There are used sensors that detect various kinds of physical quantities such as a temperature, a pressure, and a flow rate for control and safety ensuring of vehicles, consumer products, industrial equipment, etc. Such a sensor has a physical quantity detection part for taking out a physical quantity as an electric signal, an amplifier part for amplifying the electric signal to a desired magnitude, an output signal modulation part for outputting the detected physical quantity to the outside, etc. A series of signal manipulations and signal processing are performed by a logical circuit such as a microprocessor. Since microprocessors used for an automotive application support a power source voltage variation and a variation of the earth potential, many operate with a voltage of about 5V.
A sensing element for detecting a physical quantity and its peripheral circuits have advanced to a stage where, due to advances in a semiconductor technology, the detected physical quantity is converted into a numeral value through digital processing by a circuit on a single semiconductor substrate and these pieces of information are sent to a higher order circuit by reduced-wiring serial communication, for example, I2C (Inter-Integrated Circuit), etc. This is because the reduced-wiring serial communication such as the I2C can reduce the number of wiring because transmission and reception in communication are used alternately on a single strand of wiring, and eventually can miniaturize the sensor itself.
In the case where a weak signal that the sensor uses to transmit the detection result is received and a physical quantity is detected by interpreting the weak signal, there is a case where an error may be produced in the detected physical quantity by heat generation of a digital processing circuit. Therefore, a technique of reducing a consumption current by lowering an operating voltage of the circuit and making a heating value small has been widely adopted. As for the power source voltage of the sensor at this time, a voltage of about 2 V to 3 V is used.
In the case where a sensor system is comprised by connecting these serial communication type sensor and microprocessor, since bidirectional serial communication needs to be conducted between the circuits whose power source voltages differ as mentioned above, generally a bidirectional level converter circuit is used.
In FIG. 1 of the below-mentioned Patent Literature 1, a signal of a microcomputer on a 5-V side is subjected to voltage division with voltage dividing resistors r2 and r3, and they are transmitted to a microcomputer of 3.3-V side through a diode d2. A signal from a 3.3-V side microcomputer is transmitted to a 5-V side microcomputer through a diode d1, and the voltage dividing resistors r2 and r3 operate as load resistors. Under this circuit configuration, when the 5-V side microcomputer is a high level, the 3.3-V side microcomputer becomes not less than 2.2 V and less than 3.3 V; when the 5-V side microcomputer is a low level, the 3.3-V side microcomputer also becomes a low level; conversely, when the 3.3-V side microcomputer becomes a high level, the 5-V side microcomputer becomes more than or equal to 3.5 V; and when the 3.3-V side microcomputer becomes the low level, the 5-V side microcomputer becomes 1.5 V. By these operations, the both of the 5-V side microcomputer and the 3.3-V side microcomputer can recognize a high level signal and a low level signal.